High Areal Density Holographic Data Storage System

ABSTRACT

An apparatus for recording or reading high areal density holographically stored information with high signal-to-noise ratio. The apparatus comprises a holographic imaging system for recording or reconstructing a holographic image, having a first numerical aperture and a first focal length and an additional optical system for filtering the signal beam, having a second numerical aperture and a second focal length, wherein the numerical aperture of the additional optical system is less than the numerical aperture of the holographic imaging system and/or the focal length of the additional optical system is greater than the optical length of the holographic imaging system.

RELATED APPLICATIONS

This application is a continuation U.S. application Ser. No. 11/295,732,filed on Dec. 6, 2005, which is a continuation of InternationalApplication No. PCT/US2004/018116, which designated the United Statesand was filed on Jun. 7, 2004, published in English, which claims thebenefit of U.S. Provisional Application No. 60/476,812, filed on Jun. 7,2003. The entire teachings of the above applications are incorporatedherein by reference.

BACKGROUND OF THE INVENTION

The need for cost-effective, high performance data storage has, for manyapplications, outpaced technology development. Enterprise storage, imagearchives, and entertainment content, among other applications, aredriving the demand for enhanced data storage solutions. Several of theseapplications currently rely on storage technologies, such as optical,magneto-optical, and magnetic tape, that use removable media. Thesetechnologies, for the most part, have relatively limited improvementsremaining on their roadmaps for attaining increased data density, orhave limitations in achievable data rates, or in random access.Holographic data storage (HDS), on the other hand, promises bothnear-term performance comparable to the most optimistic long-termprojections for these technologies, and a technology roadmap with manyyears of rapidly increasing data storage density and data transfer ratein combination with random access.

A practical embodiment of an HDS system uses relatively thin recordingmaterial, such as photopolymers, in combination with, for example, a 4foptical imaging system. Mutually coherent signal and reference beamsform an interference pattern in the volume of their overlap. A hologramis recorded when light-induced changes in the storage medium, such asphotopolymerization, produce a record of the resulting interferencepattern. Reconstruction of the recorded hologram is accomplished byfirstly illuminating the hologram with a reference beam and secondlyimaging the diffracted light onto the detector array.

Recording many independent holograms in the same volume of materialenhances data density. This process, called multiplexing, requires thateach multiplexed hologram be recorded with a unique reference beam. Manymultiplexing procedures have been described in the literature (see forexample G. Barbastathis and D. Psaltis, “Volume Holographic MultiplexingMethods”, Holographic Data Storage, H. J. Coufal, D. Psaltis, and G. T.Sincerbox (Eds.), Springer-Verlag, 2000). A particularly usefulmultiplexing procedure for relatively thin recording material uses acollimated reference beam, and combines angular and peristrophic(azimuthal) multiplexing techniques [see D. A. Waldman, H.-Y. S. Li, andE. A. Cetin, “Holographic Recording Properties in Thick Films ofULSH-500 Photopolymer”, Proceedings of SPIE, Vol. 3291, pp. 89-103(1998) and A. Pu and D. Psaltis, “High-density recording inphotopolymer-based holographic three-dimensional disks”, Appl. Optics,Vol. 35, No. 14, pp 2389-2398 (1996).

HDS systems that operate to maximize the data density, for a recordingmaterial of a particular thickness, use the highest numerical aperture(NA) lenses for the Fourier transform lens pair that said 4f opticalimaging system can accommodate. Unfortunately, the use of high NA(NA≧0.2 for HDS systems) lenses, such as in the conventional 4f opticalsystem wherein the first and second Fourier transform lens are a matchedpair and thus have identical values of NA, can introduce several factorsthat contribute to the substantial decreases of signal-to-noise (SNR) inthe HDS system. Most significantly, when high NA optics is used for thesecond Fourier transform lens, then substantially more scattered lightis imaged to the detector plane than for lower NA optics. Lightscattered from media or media substrates, along with light scatteredfrom optical and mechanical surfaces is captured more efficiently byhigh NA optics due to the shorter working distance of said lenses andthe larger acceptance field of the lens. The scattered light is imagedonto the pixilated detector and recognized as noise during hologramread-out. This phenomenon is especially evident in thinphotopolymer-based media systems where a non-90 degree interbeam anglemust be used for the recording geometry. The suppression of noise fromvarious sources is critical to the maximization of storage densities, inparticular the suppression of optical noise. A typical HDS system hasseveral potential sources of optical noise including the aforementionedlight scattered from the media and/or optical components, reflectionsfrom surfaces internal to the drive, and, additionally, imagemisalignment and distortion. In general, each of these potential noisesources become increasingly more evident and problematic in systems thatendeavor to maximize areal density of stored data.

There is a need, therefore, for an apparatus and a method that improvesareal data density while at the same time reducing optical noise at thedetector plane that is due to scattered and stray light so as to achievegood SNR at high areal density.

SUMMARY OF THE INVENTION

In one embodiment, the present invention is an apparatus for recordingor reading holographically stored information, comprising a holographicimaging system for recording or reconstructing a holographic image,having a first numerical aperture and a first focal length, and anadditional optical system for filtering the holographic image, having asecond numerical aperture and a second focal length, wherein thenumerical aperture of the additional optical system is less than thenumerical aperture of the holographic imaging system.

In another embodiment, the present invention is an apparatus for readingor writing holographically stored information, comprising an holographicimaging system, having a first focal length, for recording orreconstructing a holographic image, and an additional optical system,having a second focal length, for filtering the holographic image,wherein the first focal length is less than the second focal length.

In another embodiment, the present invention is a method of reading aholographically recorded image, comprising directing a reference beaminto a holographic imaging system that includes a holographic recordingmedia, the holographic imaging system having a first numerical apertureand a first focal length, thereby reconstructing a signal beam;directing or relaying the reconstructed signal beam through anadditional optical system, having a second numerical aperture and asecond focal length, wherein the second numerical aperture is smallerthan the first numerical aperture, thereby filtering the reconstructedsignal beam; and detecting the filtered reconstructed signal beam.

In another embodiment, the present invention is a method of reading aholographically recorded image, comprising directing a reference beaminto a holographic imaging system that includes a holographic recordingmedia, the holographic imaging system having a first focal length,thereby reconstructing a signal beam; directing the reconstructed signalbeam through an additional optical system, having a second focal length,said additional optical system configured so that the second focallength is greater than the first focal length, thereby filtering thereconstructed signal beam; and detecting the filtered reconstructedsignal beam.

In another embodiment, the present invention is a method of recording aholographic image, comprising directing a signal beam through aholographic imaging system that includes (i) an imaging lens element anda holographic recording media, spaced apart, and (ii) an aperturedfilter, disposed between the lens element and the holographic recordingmedia, thereby producing a filtered signal beam; and directing thefiltered signal beam and a reference beam at the holographic recordingmedia, thereby recording a pattern of interference between the filteredsignal beam and the reference beam.

In another embodiment, the present invention is an apparatus for readinga holographically recorded image comprising (i) means for directing areference beam into a holographic imaging system that includes aholographic recording media, said holographic imaging system having afirst numerical aperture and a first focal length, therebyreconstructing a signal beam; (ii) means for directing the reconstructedsignal beam through an additional optical system, having a secondnumerical aperture and a second focal length, wherein the secondnumerical aperture is smaller than the first numerical aperture, therebyfiltering the reconstructed signal beam; and (iii) means for detectingthe filtered reconstructed signal beam.

In another embodiment, the present invention is an apparatus for readinga holographically recorded image, comprising means for directing areference beam into a holographic imaging system and reconstructing asignal beam, the holographic imaging system including a holographicrecording media, said holographic imaging system having a first focallength; means for directing the reconstructed signal beam through anadditional optical system and filtering the reconstructed signal beam,said additional optical system having a second focal length andconfigured so that the second focal length is greater than the firstfocal length; and means for detecting the filtered reconstructed signalbeam.

In another embodiment, the present invention is an apparatus forrecording a holographic image, comprising means for directing a signalbeam through a holographic imaging system for filtering the signal beam,the holographic imaging system including an imaging lens element and aholographic recording media, spaced apart, and an apertured filterdisposed between the lens element and the holographic recording media;and means for directing the filtered signal beam and a reference beam atthe holographic recording media for recording a pattern of interferencebetween the filtered signal beam and the reference beam.

In another embodiment, the present invention is a method of recording aplurality of holographic images in a holographic media, comprisingrecording a first of a plurality of multiplexed holograms in at leastone storage location on the holographic recording media with a firstinterbeam angle; and recording another of a plurality of multiplexedholograms in said storage location on the holographic recording mediawith a second interbeam angle. The second interbeam angle is larger thansaid first interbeam angle and wherein the multiplexed holograms at saidstorage location are partially or fully overlapped.

In another embodiment, the present invention is a method of reading aplurality of holographic images recorded in a holographic media. Themethod comprises (i) directing a reference beam at a storage location ina holographic recording media at a first incidence angle adjusted by afirst adjustment angle, thereby reading a first of a plurality ofmultiplexed holograms in at the least one storage location; (ii)directing a reference beam at said storage location on the holographicrecording media at a second incidence angle adjusted by a secondadjustment angle, thereby reading at least a second of the plurality ofmultiplexed holograms recorded in said at least one storage location;and (iii) detecting the first and at least the second holograms. In thisembodiment of the present invention, the first and the second adjustmentangles are substantially equal.

The devices and methods of the present invention achieve high arealdensity (≧24 bits/μm²) with acceptable SNR (e.g. SNR corresponding to araw-bit-error-rate (BER) of not greater than about 10⁻²) for storedinformation that comprises multiplexed holograms that have diffractionefficiency of about 10⁻³ or less.

The invention further provides a method and apparatus for utilizingmoderately high (>0.2) to high NA (≦0.85) components in HDS systems, soas to achieve high areal density of stored information (≧24 bits/μm²)when the recording geometry of the reference beam and object beamcomprises a non 90 degree interbeam angle, such as would be the case forphotopolymerizable, or photochromic, or organic photorefractiverecording media.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other objects, features and advantages of theinvention will be apparent from the following more particulardescription of preferred embodiments of the invention, as illustrated inthe accompanying drawings in which like reference characters refer tothe same parts throughout the different views. The drawings are notnecessarily to scale, emphasis instead being placed upon illustratingthe principles of the invention.

FIG. 1 is a schematic diagram of a traditional 4f optical design of aholographic recording system.

FIG. 2 is a schematic diagram of one embodiment of an apparatus of thepresent invention.

FIG. 3 depicts a portion of a device of FIG. 2 where the holographicrecording media has been offset. It shows a schematic diagram ofexemplary optical trajectories of the diffracted and undiffractedportions of a reference beam within a 4f-like optical design.

FIG. 4A is a schematic diagram showing pixel misregistration due tomisalignment of the imaged pixels and the pixels of the detector.

FIG. 4B is a schematic diagram showing an exemplary result of filteringoptical noise due to pixel misregistration using the apparatus of thepresent invention.

FIG. 5A and FIG. 5B are schematic diagrams of embodiments of the deviceof the present invention that include an additional apertured filter.

FIG. 6 is a schematic diagram illustrating one embodiment of anapparatus of the present invention for recording a holographic image.

DETAILED DESCRIPTION OF THE INVENTION

The elements of a 4f optical design are presented in FIG. 1. Signal beam30 passes through a two dimensional array of substantially transparentor opaque pixels that are formed by the spatial light modulator (SLM) 1.Alternatively, a one dimensional array of substantially transparent oropaque pixels may be used, or the signal beam may instead reflect from atwo dimensional or one dimensional array of pixels that are formed bythe SLM. SLM 1 encodes signal beam 30 with data information that is tobe recorded. The SLM typically modulates either the amplitude or thephase of an incident light beam, and can operate by transmission, asshown in FIG. 1, or by reflection of the incident beam to encode thedata information in signal beam 30. A 4f optical arrangement of FIG. 1comprises Fourier transform lens element (a first imaging lens element)2, having focal distance f1, that operates to relay a Fourier transformof SLM-encoded signal beam 30 to plane 21 that is one focal distance f1away from first imaging lens element 2. Plane 21 is two focal distancesf1 away from the SLM. Second Fourier transform lens element (secondimaging lens element) 3, which is generally referred to as a matchedpair with element 2, is positioned at (2×f1)+f2 (i.e. 3×f1 when f1=f2)distance from SLM 1 and operates to reconstruct an image of SLM 1 atplane 15 that is one focal distance f2 away from second imaging lenselement 3 and corresponds to the correlation plane at detector array 4.Plane 15 is referred to as the “image plane” or as the 4f-plane whenf1=f2. As the name suggests, in a 4f optical system, plane 15 is fourfocal distances f1 away from SLM 1 when f1=f2. The 4f system is designedfor 1:1 imaging of SLM 1 onto detector 4.

As used herein, the term “lens element” refers to one or more elementshaving optical power, such as lenses, that alone or in combinationoperate to modify an incident beam of light by changing the curvature ofthe wavefront of the incident beam of light. Lens elements 2 and 3, forexample, can comprise more than one lens. One skilled in the art willappreciate that FIGS. 1 and 2 are not drawn to scale and do not depictactual ray trajectories within the lens elements.

The cross-sectional area of signal beam 30 is typically minimized inFourier (focal) plane 21. In accordance with the fundamental Nyquistaperture for coherent light, the cross-sectional area has a diameter ofd=2λf/δ, where λ is the wavelength of the light, f is the focal distanceof first Fourier transform lens element 2, and δ₁ is the pitch of SLM 1.As used herein, the pitch of a pixel array is defined as acenter-to-center distance between two adjacent pixels. Positioningrecording material 8 at or near the focal plane 21 of Fourier transformlens element 2 typically minimizes the image size of the recorded areaand, therefore, maximizes the resulting areal data density. Recordingholograms at fractional Fourier planes that are in front of or behindthe Fourier plane and are near the Fourier plane, however, can improvethe fidelity of recorded information substantially (see G. Goldman,Optik, Vol. 34, No. 3, 254-267 (1971)) due to there being a more uniformintensity distribution of the object field over the recording area (i.e.the amplitude of the intensity variation of the object field, whichcomprises the components of the Fourier spectrum as a function ofdistance from the center of the Fourier transform, is reduced when thedistance between the Fourier plane and the recording plane isincreased).

A convenient way to carry out multiplexing can be understood withreference to FIG. 1. In FIG. 1 optical axis 34 of signal beam 30 isincident normal to the plane of recording material 8. Alternatively, theoptical axis 34 of the signal beam can be incident at oblique angles tothe normal to the plane of the recording material 8. A collimatedreference beam (9 or 10) is incident to the plane of the recordingmaterial 8 with an angle of incidence θ, that typically is a relativelylarge angle of incidence, so as to overlap signal beam 30 in the planeof recording material 8. A series of angle-multiplexed holograms can befirst recorded in the same volume of medium 8 at a selected storagelocation. Each hologram in this series is recorded with a reference beamthat has a distinct angle of incidence with respect to the normal to therecording material 8, such as, for example, reference beams 9 or 10. Byway of example, after each series of angle-multiplexed holograms iscomplete or partially complete in said selected storage location,recording material 8 is rotated by several degrees about optical axis 34of signal beam 30 or alternatively the optical axis of reference beam 9or 10 is rotated with respect to optical axis 34 of signal beam 30 sothat the plane containing said reference and signal beams is a differentplane rotated with respect to the former (azimuthal or peristrophicmultiplexing), and a new angle multiplexing series or partial series isrecorded. Alternatively, a sequence of recordings can be implementedfirst for a group of rotation angles of the recording material about theoptical axis 34 of signal beam 30 or alternatively about an axis that istilted with respect to optical axis 34 of signal beam 30 oralternatively the optical axis of reference beam 9 or 10 is rotated withrespect to optical axis 34 of signal beam 30 so that the planecontaining said reference and signal beams is a different plane rotatedwith respect to the former, corresponding to azimuthal or peristrophicmultiplexing, for a distinct angle of incidence θ of reference beam 9(or 10) with respect to the normal to recording material 8. Then,recording can be repeated for each distinct angle of incidence θ of areference beam for each of the angles of azimuthal multiplexing.Alternatively, the angles for the series of azimuthally multiplexedholograms can be partially complete for a distinct angle of incidence θof a reference beam and one or more of a series of angle-multiplexedholograms can be recorded each at another distinct angle of incidence θof a reference beam. Several hundred independent holograms can bemultiplexed in relatively thin recording material using this combinationof multiplexing procedures; resulting data densities can exceed 100 bitsper square micron (see D. Waldman et al., “CROP holographic storagemedia for optical data storage greater than 100 bits/μm²”, OrganicHolographic Materials and Applications, Vol. 5216-1, SPIE AnnualMeeting, San Diego, August 2003). Implementation of the above-describedcombination of multiplexing procedures achieves highest areal densityfor a given thickness of the recording material when the range ofinterbeam angles (i.e. angles between signal beam 30 and any one ofreference beams 9 or 10) used for the angle multiplexing procedure isthe largest that the optical system can accommodate. Consequently, it ismost practical to use the smallest interbeam angle possible (i.e. thesmallest interbeam angle for the collimated reference beam that clearsthe lens element relaying the object beam), so as to be able to recordwith the largest number of distinct angles of incidence θ of thereference beam and thus record the largest number of multiplexedholographic images in a selected storage location. It is preferable thatthe sequence of interbeam angles used for angle-multiplexing in aselected storage location be such that the smallest interbeam anglesoccur at the beginning of the recording sequence and the largest at theend of the recording sequence.

Increasing areal density can be achieved in a number of ways. Areal datadensity is increased as the total number of pixels in the SLM data pageincreases, with focal distance f1 of first Fourier transform lenselement 2 and the pixel pitch δ₁, remaining the same (see FIG. 1). Arealdata density is increased as the size of Nyquist aperture of the Fouriertransform of the SLM in the plane of the recording material 8 decreases,such as by diminution of the focal distance for first Fourier transformlens element 2 (as well as by increasing pitch for the pixels of SLM 1).Increasing the number of multiplexed holograms recorded in the samestorage location also increases the areal density.

The numerical aperture (NA) of the first Fourier transform lens element2 can be increased while maintaining the same optical field with thesame clear aperture to accommodate the same page size, and in thismanner decrease said focal distance from the said first lens to theplane of the media. This approach is generally required and typicallyimplemented to reduce the cross sectional area of the signal beam in therecording plane and thereby increase the areal density of the storagelocation.

Consequently, an HDS system that operates to maximize the data density,for a recording material of a particular thickness, uses the highest NAlenses for the Fourier transform lens pair that a 4f optical imagingsystem can accommodate. Unfortunately, the use of high NA (NA≧0.2 forHDS systems) lenses, such as in the conventional 4f optical systemwherein the first and second Fourier transform lens element havesubstantially equal focal lengths and values of NA, can introduceseveral factors that contribute to the substantial decreases ofsignal-to-noise ration (SNR) in the HDS system. Referring again to FIG.1, when NA of second Fourier transform lens element 3 is increased, sois the amount of scattered light that is imaged onto detector plane 15.Light scattered from media 8 or media substrates 6 and 7, along withlight scattered from optical and mechanical surfaces of the assembly, iscaptured more efficiently by high NA optics due to the shorter workingdistance (i.e. the distance between the rear surface of the lens and thefocal plane of the lens) of these high NA lenses and their largeracceptance field. During recording, the scattered light can be recordedin the media, and during hologram read-out the scattered light is imagedonto pixilated detector 4 and recognized as noise. This phenomenon isespecially evident in thin photopolymer-based media systems where anon-90 degree interbeam angle must typically be used for the recordinggeometry. A 90 degree interbeam can be realized for the recordinggeometry in thin photopolymer-based media systems, but this wouldrequire, by way of example, coupling the light for recording into themedia through facets that are part of the surface of the substrates ofthe media or of the recording material.

The optical noise level attributed to scattered light, ε_(scatter),measured in terms of diffraction efficiency (i.e. a ratio of intensitiesof the diffracted light and the impinging light), η, scales as thesquare of the NA as shown in Eqn (1).

η_(scatter)≈4NA²ε_(scatter)  (1)

In order to maximize the density of data stored, however, HDS systemsmust maximize the signal-to-noise ratio (SNR) of the media/drive system,where SNR is defined as

$\begin{matrix}{{S\; N\; R} = \frac{\mu_{1} - \mu_{2}}{\sqrt{\sigma_{1}^{2} + \sigma_{2}^{2}}}} & (2)\end{matrix}$

where μ₁ and μ₂ are the means of the intensity values detected, and σ₁and σ₂ are the standard deviations of the intensity values detected, forbinary zero and binary one values, respectively. Maximizing SNR,however, becomes increasingly difficult as NA is increased so as toachieve the highest storage density for the HDS system.

To circumvent the problems of noisy optical systems, large diffractionefficiencies (a physical value related to brightness or signal strengthof each hologram) are required for each of the multiplexed holograms inorder to achieve acceptable SNR ratios. The dynamic range (a physicalvalue related to the maximal number of detectable holograms that can berecorded), ν_(M), for a total of M multiplexed holograms is, however,limited for most practical recording materials. The dynamic range can beexpressed as shown in Eqn. (3):

$\begin{matrix}{v_{M} = {\sum\limits_{i = 1}^{M}\sqrt{\left( \eta_{i} \right)}}} & (3)\end{matrix}$

where η_(i) is a diffraction efficiency if the i-th hologram and where

${\left. \eta_{i} \right.\sim\frac{1}{M^{2}}}.$

It follows, therefore, that as one increases the maximum number ofholograms M stored at any one location so as to increase areal datadensity, the diffraction efficiency of each one hologram decreases,reducing the SNR. Accordingly, to effectively maximize areal datadensity by combining use of high NA Fourier transform lenses and largenumbers of multiplexed holograms, each of relatively low diffractionefficiency, it is necessary to suppress optical noise at the detectorplane.

The disclosed invention is an apparatus for holographic data storage(HDS) systems that comprises optical components such as one or morelenses and/or one or more mirrors having moderately high to high numericaperture (NA), defined for HDS systems as NA of not less than 0.2. Thesystem achieves high areal density of stored information, defined as notless than 24 bits/μm², with acceptable signal-to-noise ratio (SNR),defined as SNR corresponding to a raw bit-error-rate (BER) of ≦10E-2. Inone embodiment, the stored information comprises multiplexed holograms.The apparatus of the present invention comprises an optical system thatcan improve areal data density in holographic data storage systems byreducing optical noise at the detector. The optical noise can originate,for example, due to scattered and stray light, light from theundiffracted reference beam and image misalignment.

The apparatus of the present invention operates to achieve high arealdensity with acceptable SNR independent of the recording method used torecord the holograms and independent of the type of reference beam (i.e.collimated, spherical, elliptical, speckle, phase-code, etc.) used forrecording the holograms. If the multiplexed holograms are used, they canbe recorded by any method known to one skilled in the art including, butis not limited to, in-plane, out-of-plane angle or peristrophic(azimuthal) multiplexing, or in-plane or out-of-plane shiftmultiplexing, spatial, wavelength, phase-coded or correlationmultiplexing, or combinations thereof. The method and apparatus of thepresent invention can further be used to achieve high areal density ofstored information with acceptable SNR when the recording geometry ofthe reference beam and object beam comprises a non-90 degree interbeamangle, which would typically be the case for photopolymerizable,photochromic, or organic photorefractive recording media. The method andthe apparatus of the present invention can be utilized to reconstructand detect, with acceptable SNR, multiplexed holograms havingdiffraction efficiency of 10⁻³ or less, stored at high areal density aswell as to record such holograms.

Referring to FIGS. 1 and 2, if the NAs of lens elements 2 and 3, usedduring recording in the holographic imaging system, are sufficientlylarge, then significant optical noise degrades the SNR of the diffractedimage. The apparatus and the methods of the present invention cansubstantially reduce or eliminate optical noise. The optical noise canoriginate from various sources including, but not limited to: lightscattered from the media and other optical surfaces, light thatoriginates from the undiffracted reference beam, and image misalignment.The noise contribution from any one of these sources can cause asignificant decrease in SNR levels during hologram read-out, and can beespecially problematic when reading holograms that were recorded at highareal density, since such holograms typically have low diffractionefficiency. Without being limited by any specific theory, it is believedthat primary sources of noise include, but are not limited to lightscattered by the components of the system in the direction of detector4, including scattered light from the media, substrates and otheroptical surfaces, other sources of stray light in the HDS system such aslight from reflections off of optical surfaces and mounting fixtures,and undiffracted reference beam 10 (or 9).

In one embodiment, the present invention is an apparatus for reading orwriting holographically stored information, comprising a holographicimaging system for recording and reconstructing a holographic image,having a first numerical aperture and a first focal length and anadditional optical system for filtering a holographic image, having asecond numerical aperture and a second focal length. The additionaloptical system can also be referred to as an “optical noise filter”. Inone embodiment, the numerical aperture of the additional optical systemis less than the numerical aperture of the holographic imaging system.In another embodiment, the focal length of the additional optical systemis greater than the focal length of the holographic imaging system. Inyet another embodiment, the numerical aperture of the additional opticalsystem is less than the numerical aperture of the holographic imagingsystem and the focal length of the additional optical system is greaterthan the focal length of the holographic imaging system. As used herein,when applied to an optical system or a combination of lenses, the terms“numerical aperture” and “focal length” mean effective numericalaperture and effective focal length, respectively, of such an opticalsystem or a combination of lenses. The numerical aperture or the focallength of the additional optical system are selected to substantiallyfilter out scattered light, stray light or undiffracted reference beam.

Referring to FIG. 2, the holographic imaging system of the apparatuscomprises an HDS optical system with a traditional 4f optical design(e.g. a holographic imaging system as depicted in FIG. 1). Additionaloptical system 20, shown in FIG. 2, is inserted into the traditional 4foptical system of the HDS system.

The traditional 4f optical system of the HDS system, also referredherein as a holographic imaging system, comprises first and secondimaging lens elements 2 and 3 and can further include spatial lightmodulator (SLM) 1, holographic media 5 and detector 4.

Additional optical system 20 includes first and second additional lenselements 11 and 12 and an optional apertured filter 13 having aperture14. Additional optical system 20 operates during readout ofholographically stored information to substantially reduce or eliminatesources of optical noise that are inherent to HDS systems withmoderately high and high NA.

In one embodiment, aperture 14 of apertured filter 13 is adjustable. Anexample of an adjustable aperture is an iris diaphragm. In oneembodiment, the size and/or the shape of aperture 14 are selected tosubstantially filter out scattered light, stray light, or undiffractedreference beam.

In the embodiment shown in FIG. 2, additional optical system 20 isinserted between component 3 (the second imaging lens element) andcomponent 4 (the detector) of a holographic imaging system forreconstructing a holographic image.

In one embodiment, the holographic imaging system, is a 4f systemdesigned for 1:1 imaging of SLM 1 onto detector 4. In anotherembodiment, the holographic imaging system does not comprise aconventional 4f optical design and provides for non-1:1 imaging of SLM 1onto detector 4. This would be desirable in certain cases when pixelsize of SLM 1 differs from pixel size of detector 4.

In one embodiment (see FIG. 2), apertured filter 13 is positioned at ornear focal plane 16 of first additional lens element 11 and can operateas a spatial filter. The size and the shape of aperture 14 can beadjusted. In one embodiment, apertured filter 13 is an iris diaphragm.Referring again to FIG. 2, in one embodiment, additional optical system20 comprises a 4f optical system. The object plane of additional opticalsystem 20 is positioned to coincide with image plane 15 of theholographic imaging system. First additional lens element 11 of theadditional optical system has a focal length f3 and second additionallens element 12 of the additional optical system has a focal length f4.In one embodiment, f3=f4, thereby providing 1:1 imaging of SLM 1 ontodetector 4. In another embodiment, f3≠f4. In one embodiment, first andsecond additional lens elements 11 and 12 are substantially telecentric.

Turning again to the embodiment shown in FIG. 2, the holographic imagingsystem includes first and second imaging lens elements 2 and 3 havingfocal lengths f1 and f2, respectively. In a preferred embodiment, f1=f2,thereby providing for 1:1 imaging of SLM 1 onto detector 4. In oneembodiment, f1=f2 and f3=f4. In one embodiment, first and second imaginglens elements 2 and 3 are substantially telecentric.

As described above, in a preferred embodiment of the present invention,either the numerical aperture of the additional optical system is lessthan the numerical aperture of the holographic imaging system or thefocal length of the additional optical system is greater than the focallength of the holographic imaging system or both. Accordingly, in oneembodiment, f3 is greater than f2.

A number of possible combinations of focal lengths may be used for f1,f2, f3, and f4. For instance the first and the second imaging lenselements can be separated by a distance of f1+f2 along the optical pathof the signal beam, or by a distance that is not equal to the sum off1+f2 along the optical path of the signal beam such as may be neededdue to tolerance specifications of optical elements. Additionally, thefirst and the second additional lens elements can be separated by adistance of f3+f4 along the optical path of the signal beam, or by adistance that is not equal to the sum of f3+f4 along the optical path ofthe signal beam such as may be needed due to tolerance specifications ofoptical elements. For any combination of these conditions other possibleoptical arrangements are possible such as f1=f2 or f1≠f2 or f3=f4 orf3≠f4, and any combinations thereof, such as the arrangement comprisingf1=f2 and f3=f4 when f3>f2, or f1=f2 and f3≠f4 when f3>f2 such as may beneeded to provide magnification or demagnification.

Holographic recoding media 5 is positioned at or near Fourier transform(focal) plane 21 of first imaging lens element 2. Holographic recordingmedia 5 comprises, by way of example, a layer of photopolymerizablematerial 8 disposed between two optically transmissive planar substrates6 and 7. The thickness of the holographic recording material istypically between about 200 μm and 1.5 mm.

In one embodiment of the present invention, it is desirable to magnifyor demagnify the SLM image. This may be desired when pitch δ₁ of thepixels of spatial light modulator 1 is not equal to the pitch or somewhole number factor of the pitch of pixels δ₂ of detector 4 (see FIG.2). In this embodiment, the optical components of the additional opticalsystem 20, such as lens elements 11 and/or 12, can magnify or demagnifythe image of SLM 1 to correctly project it onto detector 4.

In another embodiment, the apparatus of the invention further includes asecond apertured filter disposed along optical path 34 of signal beam30. In one embodiment, shown in FIG. 5A, second apertured filter 60 isdisposed between first and second additional lens elements 11 and 12along optical path 34. In this embodiment first apertured filter (13)operates as a phase contrast filter and said second apertured filter(60) operates as a noise filter. In another embodiment, shown in FIG. 5Band, partially, FIG. 6, the second apertured filter 70 is disposedbetween first imaging lens element 2 and holographic recording media 5.This configuration, is particularly useful for filtering out higherorders of the amplitude distribution of the Fourier transform at therecording plane so as to prevent undesirable consumption of the dynamicrange of the recording material outside of the storage location.

During typical holographic recording of digital data pages, an imagecomprising data information for the signal beam is displayed by spatiallight modulator (SLM) 1. The SLM can operate to modulate either theamplitude or the phase distribution of an impinging laser beam (here,beam 30; see FIG. 2). The SLM can be reflective or transmissive, thelatter type shown schematically in FIGS. 1 and 2. The modulated signalbeam (beam 30) is focused by Fourier transform lens element (firstimaging lens element) 2, that operates to relay the optical Fouriertransform of the SLM pattern comprising data information to plane 21located one focal length f1 from first imaging lens element 2. Referencebeam 9 or 10, coherent with signal beam 30, propagates towards andinteracts with signal beam 30 at selected storage location atphotopolymerizable recording material 8. One skilled in the art ofholographic data storage will appreciate that there is a plurality ofaddressable storage locations in media 5. An interference pattern isformed by the overlap of signal beam 30 and reference beam 10 withinmedia 8. Several to many co-locational or partially overlappingholograms can be recorded in a selected storage location using variousmultiplexing techniques that are readily apparent to those skilled inthe art.

Reconstruction of a hologram for reading is schematically depicted inFIG. 3. FIG. 3 represents a portion of a device of FIG. 2 with aposition of media 5 offset by a distance d with respect to focal plane21. The read-out of a hologram or a series of holograms requires thatreference beam 10, substantially identical to the recording referencebeam 9 or 10 (see FIG. 2), impinge upon the selected hologram ofinterest. The reference beam diffracts from the diffraction grating 52(formed in media 8 during recording) with a diffraction efficiency thatgenerally depends upon the thickness of the hologram, the refractiveindex modulation of the hologram, and the recording geometry. Thediffracted light forms image cone 50 and propagates through the inverseFourier transform lens element (second imaging lens element) 3 of theholographic imaging system. An image from the recorded interferencepattern is relayed to image plane 15 by the inverse Fourier transformlens element (second imaging lens element) 3. The holographic imagingsystem can further include a detector, such as detector 4 in FIG. 2.Absent the additional optical system of the present invention, detector4 can be placed at image plane 15, as shown in FIG. 1.

However, a portion of the impinging reference beam, depicted as beam 54in FIG. 3, does not diffract and can further be collected by secondimaging lens element 3. Additionally, scattered light from varioussources that include the undiffracted portion 54 of reference beam 10 aswell as other optical noise can be collected by second imaging lenselement 3 and degrade the SNR. (These other sources of optical noise arenot shown in FIG. 3, but are discussed below.)

Accordingly, in one embodiment, the present invention is a method ofreading a holographically recorded image. The method comprises directingreference beam 9 or 10 into a holographic imaging system that includesholographic recording media 5, at a selected storage location on media 5comprising one or more holographically recorded images, therebyreconstructing a signal beam. The holographic imaging system has a firstnumerical aperture and a first focal length. The method further includesdirecting the reconstructed signal beam through additional opticalsystem 20, having a second numerical aperture and a second focal length.The second numerical aperture is smaller than the first numericalaperture. In passing through additional optical system 20, thereconstructed signal beam is filtered. The filtered signal beam isdetected by detector 4.

In one embodiment, the holographic recording media stores fully orpartially overlapped multiplexed holograms in at least one storagelocation on the holographic recording media. These modes of multiplexingare carried out by aforementioned methods such as either changing theincident angle of the reference beam or by moving the holographicrecording media by a distance that is less than the characteristic sizeof a storage location. In another embodiment, the multiplexed hologramsare recorded so that the first hologram in a sequence of multiplexedholograms is recorded with a smaller interbeam angle than hologramsrecorded later in the sequence.

Referring to FIG. 2, in one embodiment, additional optical system 20blocks a significant percentage of light scatter that is from outsideimaging cone 50 of the holographic imaging system. Those skilled in theart can select from a variety of methods to increase working distance ofthe optical system of the present invention so as to reduce thecontributions of noise that otherwise are imaged onto detector 4.

Referring to FIG. 3, a practical holographic imaging system willgenerally have several sources of optical noise from scattered light.These include light scattered from media substrates 6 and 7, recordingmaterial 8, surfaces of lens elements 2 and 3, and mechanical fixtures(not shown). An appreciable proportion of this noise from scatteredlight can arise from the polymer structure in the recording materialand/or polymeric substrates and/or from inclusions or bubbles in glasssubstrates. Consequently, the intensity of forward scattered light hascontributions that are not spherically symmetrical and depends on θ, theangle between the continued propagation direction of the undiffractedreference beam 9 or 10 and the direction of propagation of scatteredlight, as 1/sin²(θ/2) or (1+cos² θ)/2. Preferably, the additionaloptical system 20 blocks forward-scattered light in the range of aboutθ=0° to about ±5°, more preferably, in the range of about θ=0° to about10°, and even more preferably about θ=0° to about ±20° from thepropagation direction of the undiffracted reference beam 9 or 10. Thisis achieved by selecting NA and/or focal lengths of lens elements 11 and12 as well as selecting the size of the spatial filter 13 and the sizeof aperture 14.

Referring to FIG. 3, when using amplitude-modulated SLM, the Fouriertransform spectra of the SLM image typically includes a substantiallydominant constant component (so called DC component) that is capable ofover-exposing the photopolymer media during such recording. Therefore,when recording is made using an amplitude-modulated SLM, it is typicallynecessary to position recording media 5 away from Fourier plane (focalplane) 21 along optical axis 34 in order to homogenize the amplitudedistribution of the Fourier transform of the object field and achieveacceptable SNR. The offset is shown as distance d behind the Fourierplane 21 in FIG. 3, which is a preferable geometry that provides forusing the minimum interbeam angle for Reference beam 9 or 10 and thusthe smallest nominal slant angle as well as the largest available rangeof reference beam angles. This results in undiffracted portion 54 ofreference beam 9 or 10 being able to enter inverse Fourier transformlens element (the second imaging lens element) 3 of the holographicimaging system.

One skilled in the art will appreciate that during reconstruction of ahologram recorded with such an offset, the media must be placed at thesame offset position as well. Consequently, in a one embodiment of theapparatus and method of this invention media 5 is offset from Fourierplane (focal plane) 21 of the holographic imaging system by distance d.In a more preferred embodiment, recording media 5 is positioned behindthe Fourier plane (focal plane) 21 along optical axis 34 in thedirection closer to inverse Fourier transform lens element (secondimaging lens element) 3. As a result, a portion of the undiffractedreference beam 9 or 10 can enter the second imaging lens element 3.

By selecting NA and/or focal lengths of lens elements 11 and 12 as wellas selecting the size of apertured filter 13 and size of aperture 14,undiffracted reference beam 54 is prevented from being imaged ontodetector 4.

It can also be advantageous to minimize the mean angle between thereference beam 9 or 10 and the signal beam 30, referred to as theinterbeam angle, in order to reduce the effect of transverse (thicknessdirection) shrinkage on image fidelity (see FIG. 2) during recording.Such shrinkage occurs during holographic recording in photopolymerizablemedia. This is particularly the case when implementing planar-anglemultiplexing or a combination of it with azimuthal multiplexing.Firstly, diminishing the interbeam angle causes thefull-width-half-height (FWHH) of the angular selectivity curve tobroaden for a particular thickness of the recording material. Since themagnitude of transverse shrinkage will be largest for the first hologramrecorded in photopolymerizable recording media, in a sequence ofplanar-angle multiplexed holograms that are recorded in the same storagelocation, the angular deviation of said first hologram from the Braggrecording condition will be larger than for holograms recorded later inthe sequence. It is therefore desirable to record first holograms withthe smallest interbeam angle. This will allow to offset larger angularshifts from the Bragg condition of recording with the largest FWHH ofthe corresponding angular selectivity profile. Secondly, the angulardeviation for the smaller interbeam angle, for a given shrinkage level,will be smaller due to the hologram having a smaller slant angle.Thirdly, diminishing the minimum interbeam angle increases the overallrange of reference beam angles that can be used to implementplanar-angle multiplexing, and thus provides the means to achieve aslarge a multiplexing number as possible for a given thickness of therecording material. Fourthly, photopolymerizable media is generallypre-conditioned to pre-shrink the media to the correct start statebefore recording information that comprises multiplexed holograms that,by way of example, are digital data page holograms This pre-conditioningreduces the dynamic range that could otherwise be used for recordinginformation. Minimization of the interbeam angle reduces the effect ofvolume shrinkage allowing one to reduce the extent of pre-conditioningand preserve dynamic range for recording information.

Therefore, in a preferred embodiment of the apparatus of this invention(see. FIG. 3) recording media 5 is positioned behind Fourier plane(focal plane) 21, and, if using angle multiplexing, the mean interbeamangle (angle θ_(r) in FIG. 3) is reduced to the smallest angle thatclears the optical components of the device. These two advantageousconditions, however, can cause a large portion of the undiffractedreference beam 54, as well as light scattered at angles close to thecontinued propagation direction of reference beam 10, to enter inverseFourier transform lens element (second imaging lens element) 3 of theholographic imaging system.

The problem of preventing undiffracted reference beam from being imagedonto the detector does not arise in the case of low areal data densitysystems. In such systems, lens elements have low NA and substantiallylonger working distances. Referring, by way of example, to FIG. 1, theundiffracted portion of the reference beam 9 or 10 generally does nottravel through the inverse Fourier transform lens element (the secondimaging lens element) 3. Even in cases, where the undiffracted portionof the reference beam does travel through lens element 3, the entranceangle of the reference beam 9 or 10 is larger than the entrance angle ofthe object (signal) beam 30. Therefore, after being relayed by the lenselement 3, the reference beam 9 or 10 will be imaged outside the imageof the hologram at the detector 4. In this case, the undiffractedreference beam 9 or 10 would have exited the optical system of theinvention, and, therefore the SNR of the system would not be compromisedby the undiffracted light. Similarly, a portion of light scattered fromreference beam 9 or 10 that enters the second imaging lens element 3,and that has a substantially higher intensity at angles close to thecontinued propagation direction of the reference beam, will be imaged toa location outside of the image of the hologram at the detector 4. Ineither case, the high intensity portion of the scatter light essentiallyexits from the optical system and therefore the SNR of the system is notcompromised.

HDS systems comprising high NA optical components that havesubstantially short focal lengths and correspondingly short workingdistances, however, do not have enough distance between inverse Fouriertransform lens element (second imaging lens element) 3 and detector 4 toallow the undiffracted portion of the reference beam, or light scatteredfrom the reference beam at angles close to the continued propagationdirection of the reference beam, to exit the optical path beforeentering the detector in the area corresponding to informationdiffracted by the hologram. Referring, for example, to FIGS. 2 and 3,placing detector 4 at plane 15 would not allow undiffracted portion ofthe reference beam 54 or light scattered from reference beam 10 atangles close to the continued propagation direction of reference beam10, to exit the optical path before entering the detector in the areacorresponding to the image of the hologram. Consequently, the SNR of thesystem can be seriously compromised such that information comprisingholograms cannot be adequately read with reasonable SNR, and thus highareal density of stored information is not achieved.

Referring now to FIG. 2, by selecting NA and/or focal lengths of firstand second additional lens elements 11 and 12 as well as selecting thesize of apertured filter 13 and the size of aperture 14, undiffractedreference beam 9 or 10 is prevented from being imaged onto detector 4.

In one embodiment, the NA of the additional optical system is reducedrelative to the NA of the holographic imaging system. This preventsundiffracted reference beam 9 or 10, as well as light scattered fromreference beam 9 or 10 at angles close to the continued propagationdirection of the reference beam, from entering the clear aperture offirst additional lens element 11.

In another embodiment, focal length f₃ of first additional lens element11 is long enough to allow undiffracted reference beam 9 or 10, as wellas light scattered from reference beam 9 or 10 at angles close to thedirection the continued propagation of the reference beam, to exit theHDS system before being collected by first additional lens element 11.Alternatively, focal length f₃ is selected so that, even if collected byfirst additional lens element 11, undiffracted reference beam (9 or 10)and light scattered from the reference beam at angles close to thedirection the continued propagation of the reference beam are blocked byapertured filter 13. Those skilled in the art can choose a particularshape and size for the aperture 14 to optimize SNR of the HDS system forread-out of holograms, said holograms preferably multiplexed so as toachieve high storage density of information and thus having lowdiffraction efficiency.

Apertured filter 13 can be used in another embodiment to reduce noiseintroduced by data page misalignment such as misalignment on a subpixelscale. FIG. 4A shows diffracted data page 41 impinging on detector 4such that it exhibits pixel misregistration. Pixel misregistration canbe measured in fractions of characteristic pixel dimension, ΔX and ΔY.Pixel misregistration can be due to factors including, but not limitedto, media wedge, media tilt, servo inaccuracy, and/or lens misalignment.Use of high NA optical components typically amplifies the likelihood ofpixel misregistration. An apertured filter, such as apertured filter 13,can be used to reduce or substantially eliminate effects of pixelmisregistration that is on the order of a fraction of the pixeldimension. Those skilled in the art can select apertured filter typesfor use as apertured filter 13 that are particularly useful forfiltering high spatial frequency components of the data page image suchthat the SNR of the HDS system is optimized. An exemplary result ofspatial filtering using apertured filter 13 is shown in FIG. 4B. Theresulting diffraction pattern is spatially filtered, thus “rounding” thesquare pixels and reducing pixel misregistration.

Referring again to FIG. 2, in one embodiment of the present invention,apertured filter 13 can be positioned at plane 16. Preferably, plane 16coincides with a focal plane of first additional lens element 11. Theposition of apertured filter 13 as well as the size of its aperture 14can be selected to pass only the spatial frequency components of thedata page image that maximize the SNR of the selected hologram beingread-out (see for example Bernal et al., Applied Optics, Vol 37, No. 23,pp 5377-5385 (1998) and G. W. Burr and M. P. Bernal Artajona, “SystemOptimization for Holographic Data Storage”, Holographic Data Storage, H.J. Coufal, D. Psaltis, and G. T. Sincerbox (Eds.), Springer-Verlag, 2000and references contained therein). For instance, an aperture sizedesigned to sample the Nyquist sampling frequency, perhaps increased byall or some fraction of the Rayleigh criteria, achieves good SNR,whereas larger apertures can pass increased levels of noise and smallerapertures can pass decreased signal levels as well as cause interpixelcrosstalk due to insufficient sampling of higher spatial frequencies ofthe Fourier spectrum of the data page hologram(s).

In one embodiment, the apparatus of the present invention can be usedfor readout of data page holograms which were recorded with the SLMoperating in phase-modulating mode. Referring to FIG. 2, when SLM 1operates to modulate the phase of the input light for signal beam 30,recording media 5 can be positioned at object (focal) plane 21 ofimaging Fourier transform lens (second imaging lens element) 3. As aresult, the size of the recording area at storage locations withinphotopolymerizable material 8 is reduced and areal density is increased.However, for a number of reasons it is desirable to convert aphase-modulated image into an amplitude modulated image for reading.Accordingly, in one embodiment, the NA, focal lengths and/or the size ofaperture 14 of apertured filter 13 are selected so that apertured filter13 operates as a phase contrast filter to convert a phase-modulatedholographic image to an amplitude-modulated one.

In one embodiment, the present invention is a method of recording aholographic image. Referring to FIG. 6, the method comprises directingsignal beam 30 through a holographic imaging system. The holographicimaging system includes an imaging lens elements 2 and holographicrecording media 5, spaced apart along optical axis 34. The holographicimaging system further includes apertured filter 70, disposed betweenthe imaging lens element and media 5 along optical axis 34 at or nearthe front surface of media 5 so as to filter higher order spatialfrequencies of the Fourier transform of the object field and therebyprevent these from exposing the media in locations outside the selectedstorage location. The method further includes directing the filteredsignal beam and reference beam 9 or 10 at a selected storage location onthe holographic recording media 5, thereby recording a pattern ofinterference at the intersection of the filtered signal beam and thereference beam.

The present invention also relates to methods of recording and readingmultiplexed holograms that reduces or substantially eliminates the needfor variable adjustment of the incidence angle of the reference beamduring reconstruction of the multiplexed holograms relative tocorresponding angles used during recording.

A holographic recording media shrinks during recording. As a result,during reconstruction of holograms, the angle of incidence of thereference beam generally needs to be adjusted, i.e. made smaller orlarger than the incidence angle of a reference beam used for recordingof the hologram being reconstructed. Adjusting the incidence angle ofthe reference beam during reconstruction to an improved Bragg matchingcondition increases the SNR. The extent of shrinkage, however, variesfrom the first to subsequent holograms in a sequence of multiplexedholographic images. Variable adjustment of the incidence angle of thereference beam during reconstruction, relative to the correspondingangles used during recording, compensates for variable shrinkage of theholographic recording media. Variable adjustment, however, increasescomplexity of an HDS system, decreases fidelity of data recovery and isdifficult to implement.

Accordingly, one embodiment of the present invention is a method ofrecording multiplexed holograms that substantially eliminates the needfor variable adjustment during recording. In this embodiment, thepresent invention is a method of recording a plurality of holographicimages in a holographic media. The method comprises (i) recording afirst hologram of a plurality of multiplexed holograms in at least onestorage location on the holographic recording media with a firstinterbeam angle and (ii) recording another of a plurality of multiplexedholograms in said storage location on the holographic recording mediawith a second interbeam angle, wherein the second interbeam angle islarger than the first interbeam angle. The multiplexed holograms at thestorage location at issue are partially or fully overlapped. As usedherein, the term “interbeam angle” is defined as the angle between theoptical axis of the reference beam and the optical axis of the objectbeam.

In another embodiment, the present invention is a method of readingmultiplexed holograms that substantially eliminates the need forvariable adjustment. In this embodiment, the present invention is amethod of reading a plurality of holographic images recorded in aholographic media. The method comprises (i) directing a reference beamat a storage location in a holographic recording media at a firstincidence angle adjusted by a first adjustment angle, (ii) detecting andthereby reading a first of a plurality of multiplexed holograms in atthe least one storage location, (iii) directing a reference beam at saidstorage location on the holographic recording media, wherein saidstorage location may be shifted from first storage location but is atleast partially overlapped with said first storage location, at a secondincidence angle adjusted by a second adjustment angle, (iv) detectingand thereby reading at least a second of the plurality of multiplexedholograms recorded in said at least one storage location, wherein thefirst and the second adjustment angles are substantially equal. Aplurality of multiplexed holograms that are recorded in the mannerdescribed, wherein for a sequence of recordings a second interbeam angleis larger than the first interbeam angle, during read-out exhibit angleshifts from the optimum Bragg matching condition, relative to thecorresponding angles of the reference beam used to record saidmultiplexed holograms, that are in the range of 0.1° for the first suchmultiplexed hologram diminishing to about 0.06° for the last multiplexedhologram that overlaps in the storage location. Consequently, for mediawith recording thickness in the range of about 0.5 mm, a globaladjustment angle can be used during read-out for each reference beamangle wherein good SNR is achieved. When multiplexed holograms areinstead recorded such that larger interbeam angles are first and smallerinterbeam angles are last for a sequence of recordings, then the rangeof angle adjustments can exceed about 0.2° which is typically largerthan the full-width-half-height of the Bragg detuning profile. In thelatter case a global adjustment angle cannot be implemented withoutsignificantly diminishing SNR of the reconstructed holograms.

Accordingly, the present invention advantageously allows the variabilityin adjustment angles for overlapping or partially overlappingmultiplexed holograms to be less than about 0.2°, preferably less than0.1° and even more preferably less than 0.06° and more preferably lessthan 0.05°. In one embodiment, the variability in adjustment angle isabout zero.

Exemplification

Using an apparatus and method of this invention, an areal density ofabout 75 to 100 bits/μm² has been recorded. The holographic recordingmedia used was Aprilis HMC-050-G-15-C-300 Cationic Ring OpeningPolymerization (CROP) media of 300 micron thickness. A 4f optical systemof the type shown in FIG. 1 was used to record 75 and 100co-locationally multiplexed data page holograms, respectively, withN=262K bits/page. The numerical aperture (NA) of the 4f optical systemwas equal to 0.28 and the area of the storage location of themultiplexed holograms was 0.27 mm². Read-out of the co-locationallymultiplexed data page holograms could not be accomplished with the 4foptical system of the type shown in FIG. 1 due to excessive noisecontributions that substantially degraded the SNR of the reconstructedholograms to unacceptably low values (raw BER=10⁻¹). Read-out of theco-locationally multiplexed data page holograms, having diffractionefficiency of η≦10E-4, was, however, accomplished with acceptable SNR of≧4.5 (10⁻³≦raw BER≦6×10⁻³) without equalization for all of the data pageholograms by using the method and apparatus of this invention, such asan embodiment shown in FIG. 2 wherein the NA of the additional opticalsystem was about 0.125. Using an apparatus and method of this invention,an areal density of about 150 bits/μm was recorded in AprilisHMC-050-G-15-C-400 holographic recording media of 400 micron thickness.A 4f optical system of the type shown in FIG. 1 was used to record 155co-locationally multiplexed data page holograms, respectively, withN=262K bits/page. The numerical aperture (NA) of the 4f optical systemwas equal to 0.28 and the area of the storage location of themultiplexed holograms was 0.27 mm². Read-out of the co-locationallymultiplexed data page holograms could not be accomplished with the 4foptical system of the type shown in FIG. 1 due to excessive noisecontributions that substantially degraded the SNR of the reconstructedholograms to unacceptably low values (raw BER˜10⁻¹). Read-out of theco-locationally multiplexed data page holograms, having diffractionefficiency in the range of 10E-4≦η≦10E-3, was, however, accomplishedwith acceptable SNR of between 3.0 and 6.0 (1E-4≦raw BER≦6E-3) withoutequalization for all of the data page holograms by using the method andapparatus of this invention, such as an embodiment shown in FIG. 2comprising the additional optical system 20 with aperture filter 13,wherein the NA of the additional optical system was about 0.125.

While this invention has been particularly shown and described withreferences to preferred embodiments thereof, it will be understood bythose skilled in the art that various changes in form and details may bemade therein without departing from the scope of the inventionencompassed by the appended claims.

1. An apparatus for reading or writing holographically storedinformation, comprising: a holographic imaging system, having a firstfocal length, for recording or reconstructing a hologram; and anadditional optical system, having a second focal length, said additionaloptical system configured to receive a reconstructed hologram from theholographic imaging system, wherein the first focal length is less thanthe second focal length.
 2. The apparatus of claim 1 wherein theholographic imaging system includes a first imaging lens element, havingfocal length f1 and a second imaging lens element, having focal lengthf2, spaced apart; and the additional optical system includes a firstadditional lens element, having focal length f3 and a second additionallens element, having focal length f4, spaced apart.
 3. The apparatus ofclaim 2 wherein f3>f2.
 4. The apparatus of claim 3 wherein f3=f4.
 5. Theapparatus of claim 3 wherein f3≠f4.
 6. The apparatus of claim 2 furtherincluding an apertured filter disposed between the first and the secondadditional lens elements.
 7. The apparatus of claim 6 wherein theaperture of the apertured filter of the additional optical system isadjustable in position or size or shape.
 8. The apparatus of claim 2,further including: a spatial light modulator for encoding a signal beam;and a detector for detecting a holographic image, wherein the first andthe second imaging lens elements are disposed between the spatial lightmodulator and the detector.
 9. The apparatus of claim 2 wherein thefirst and second imaging lens elements of are substantially telecentric.10. The apparatus of claim 2 wherein the first and the second additionallens elements are substantially telecentric.
 11. The apparatus of claim8 wherein the additional optical system is disposed between the secondimaging lens element and the detector.
 12. The apparatus of claim 8wherein the holographic imaging system further includes a holographicrecording media disposed between the first and the second imaging lenselements.
 13. The apparatus of claim 12 wherein the holographicrecording media stores fully or partially overlapped multiplexedholograms in at least one storage location on the holographic recordingmedia.
 14. The apparatus of claim 13 wherein holograms are recorded byangle-multiplexing or by combining at least two methods of multiplexing.15. A method of reading a holographically recorded image, comprising:directing a reference beam into a holographic imaging system thatincludes a holographic recording media, said reference beam directedonto the holographic media at a storage location, and said holographicimaging system having a first focal length, thereby reconstructing asignal beam; directing the reconstructed signal beam through anadditional optical system, having a second focal length, said additionaloptical system configured so that the second focal length is greaterthan the first focal length; and detecting the reconstructed signalbeam.
 16. The method of claim 15 wherein the holographic recording mediastores fully or partially overlapped multiplexed holograms in at leastone storage location on the holographic recording media.
 17. The methodof claim 16 wherein the first multiplexed hologram in a sequencerecorded in a storage location is recorded with a smaller interbeamangle than holograms recorded later in the sequence.
 18. An apparatusfor reading a holographically recorded image, comprising: means fordirecting a reference beam into a holographic imaging system andreconstructing a signal beam, the holographic imaging system including aholographic recording media, said reference beam directed onto theholographic media, said holographic imaging system having a first focallength; means for directing the reconstructed signal beam through anadditional optical system and filtering the reconstructed signal beam,said additional optical system having a second focal length andconfigured so that the second focal length is greater than the firstfocal length; and means for detecting the filtered reconstructed signalbeam.
 19. The apparatus of claim 18 wherein the holographic recordingmedia stores fully or partially overlapping multiplexed holograms in atleast one storage location on the holographic recording media.
 20. Anapparatus for recording or reading holographically stored information,comprising: a holographic imaging system for recording or reconstructinga hologram, said holographic imaging system having a first focal length;a holographic recording media optically coupled to the holographicimaging system; a detector, optically coupled to the holographicrecording media; and an additional optical system, having a second focallength, wherein the first focal length is less than the second focallength, and further wherein the additional optical system is disposedbetween the holographic recording media and the detector.
 21. Theapparatus of claim 1, wherein the holographic imaging system includes afirst imaging lens element, having focal length f1; and the additionaloptical system includes a first additional lens element, having focallength f3, and a second additional lens element, having focal length f4,spaced apart.
 22. The apparatus of claim 21, wherein f3=f4.
 23. Theapparatus of claim 21, wherein f3≠f4.
 24. The apparatus of claim 21further including an apertured filter disposed between the first and thesecond additional lens elements, and, optionally, a second aperturedfilter disposed along the optical path of the reconstructed hologram orthe optical path of a signal beam.
 25. The apparatus of claim 21 whereinthe aperture of the apertured filter of the additional optical system isadjustable in position or size or shape.
 26. The apparatus of claim 21,further including a holographic recording media; and a spatial lightmodulator for encoding a signal beam, wherein the first imaging lenselement is disposed between the spatial light modulator and theholographic recording media.
 27. The apparatus of claim 21 wherein thefirst and the second additional lens elements are substantiallytelecentric.
 28. The apparatus of claim 26 further including a detectorfor detecting a reconstructed hologram, wherein the additional opticalsystem is disposed between the first imaging lens element and thedetector.
 29. The apparatus of claim 26, wherein the holographicrecording media stores fully or partially overlapped multiplexedholograms in at least one storage location on the holographic recordingmedia.
 30. The apparatus of claim 13, wherein holograms are recorded byin-plane or out-of-plane shift multiplexing.
 31. The apparatus of claim1, wherein the additional optical system includes a first additionallens element and a second additional lens element, spaced apart, andwherein the apparatus further includes: a spatial light modulator forencoding a signal beam; and a detector for detecting a holographicimage, wherein the first and the second additional lens elements aredisposed between the spatial light modulator and the detector.